Update Rust crate toml_edit to 0.21 (#515)

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This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [toml_edit](https://togithub.com/toml-rs/toml) | dependencies | minor
| `0.20` -> `0.21` |

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### Release Notes

<details>
<summary>toml-rs/toml (toml_edit)</summary>

###
[`v0.21.0`](https://togithub.com/toml-rs/toml/compare/v0.20.7...v0.21.0)

[Compare
Source](https://togithub.com/toml-rs/toml/compare/v0.20.7...v0.21.0)

###
[`v0.20.7`](https://togithub.com/toml-rs/toml/compare/v0.20.6...v0.20.7)

[Compare
Source](https://togithub.com/toml-rs/toml/compare/v0.20.6...v0.20.7)

###
[`v0.20.6`](https://togithub.com/toml-rs/toml/compare/v0.20.5...v0.20.6)

[Compare
Source](https://togithub.com/toml-rs/toml/compare/v0.20.5...v0.20.6)

###
[`v0.20.5`](https://togithub.com/toml-rs/toml/compare/v0.20.4...v0.20.5)

[Compare
Source](https://togithub.com/toml-rs/toml/compare/v0.20.4...v0.20.5)

###
[`v0.20.4`](https://togithub.com/toml-rs/toml/compare/v0.20.3...v0.20.4)

[Compare
Source](https://togithub.com/toml-rs/toml/compare/v0.20.3...v0.20.4)

###
[`v0.20.3`](https://togithub.com/toml-rs/toml/compare/v0.20.2...v0.20.3)

[Compare
Source](https://togithub.com/toml-rs/toml/compare/v0.20.2...v0.20.3)

###
[`v0.20.2`](https://togithub.com/toml-rs/toml/compare/v0.20.1...v0.20.2)

[Compare
Source](https://togithub.com/toml-rs/toml/compare/v0.20.1...v0.20.2)

###
[`v0.20.1`](https://togithub.com/toml-rs/toml/compare/v0.20.0...v0.20.1)

[Compare
Source](https://togithub.com/toml-rs/toml/compare/v0.20.0...v0.20.1)

</details>

---

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Author: gwilymk